1. Field of the Present Invention
The present invention generally relates to the field of data storage systems and more particularly to data storage systems that implement redundant disk arrays.
2. History of Related Art
In the field of data storage, redundant disk arrays are well known. Perhaps the most widely known implementations of disk arrays are referred to as RAID, an acronym for redundant array of independent (or inexpensive) disks. In a RAID system, multiple disks are used to store data more reliably than data can be stored on a single disk. In a single disk system, a disk crash results in the loss of any data that has not been saved to an external storage medium such as a tape. In RAID systems, the use of multiple disks improves reliability by enabling data redundancy. In the context of RAID systems, the term redundancy refers to the system's immunity to disk crashes. More specifically, RAID redundancy enables the system to recover all data following the crash of any single disk within the system.
The varieties of RAID redundancy are the subject of extensive literature. In a RAID 1 system, redundancy is achieved by “mirroring”, which is simply storing a copy of all data on two disks. Although this type of redundancy is relatively expensive in terms of data storage (because at least 50% of storage capacity is used for redundancy), RAID 1 systems are simple to implement and have performance advantages over other RAID schemes. In a RAID 4 system, data is “striped” across multiple drives to improve performance (by enabling simultaneous access to different sections of a file) while redundancy is achieved by storing parity information on a single drive (the parity drive). In a RAID 5 system, data is typically striped across multiple disks in a RAID 4 fashion, but the parity information is distributed across multiple disks such that a portion is stored on a first disk, a portion on a second disk, and so forth. RAID 5 systems allow simultaneous access to different regions of parity information, which can further improve performance.
Historically, the primary considerations given to the design of RAID systems were performance and storage cost. Performance, in turn, was typically optimized by maintaining all disks in an active state to minimize access delays. More recently, an increasingly important consideration in the design of RAID systems is operating cost or energy consumption.
Disk drives contain both mechanical and electronic components that consume energy. The mechanical components rotate the drive platters under the read/write heads and position the heads over a specific track of the platter. The electronic components accept commands from a device bus and process the commands by directing the operation of the mechanical components as necessary.
In current technology disk drives, the majority of the drive's energy consumption is attributable to the drive's mechanical components and, in particular, to the rotational motor responsible for spinning the platters. Consequently, power management techniques for disk drives typically attempt to conserve energy by turning off the rotational motor(s) during periods when the drive is not processing commands. Unfortunately, the performance cost of turning off the drive motors, measured in terms of the time required to return the drive to the active state in which data can be read or written from the disk, is significant (typically 2 to 5 seconds). Thus, in an implementation that aggressively spins down disks to conserve power consumption, performance degradation is a significant issue.
Accordingly, it would be desirable to implement a data storage system and methodology that addressed the issues of energy consumption and performance in a redundant disk array.